The present invention relates to an optical element having an antireflection film on a plastic substrate. It further relates to an optical element having an antireflection film on a plastic substrate that has good heat resistance.
Optical elements having an antireflection film formed on a plastic substrate were known heretofore. For example, Japanese Patent Laid-Open No. 291501/1990 discloses an optical element formed with an antireflection film that has a high-refraction layer of xcex/2 containing titanium dioxide as a major component.
In general, however, the heat resistance of such optical elements having an antireflection film provided on a plastic substrate is not good compared with that of optical elements comprising an antireflection film provided on a glass substrate, because the former cannot be heated during vapor deposition. Therefore, there is a need for optical elements having an antireflection film provided on a plastic substrate having improved heat resistance.
The inventors have determined that the heat resistance of an optical element with a plastic substrate is significantly improved by using an equivalent film of at least three layers, while utilizing a high-refraction layer and a layer made of a low-refraction substance such as silicon dioxide.
Heretofore, the high-refraction layer of xcex/2 was constructed of a single layer of a high-refraction vapor-deposited substance such as titanium dioxide, zirconium oxide or tantalum oxide. A single layer provided the antireflection property and promoted production efficiency. In this connection, if the high-refraction layer of xcex/2 is provided with a layer made of a low-refraction substance, such as silicon dioxide, it will lower the refractive index of the high-refraction layer and therefore possibly lower the antireflection property of the antireflection film. For these reasons, such a construction has not heretofore been proposed.
The invention provides an optical element having an antireflection film that comprises a plastic substrate and an antireflection film of a xcex/4-xcex/2-xcex/4 or xcex/4-xcex/4-xcex/2-xcex/4 type (xcex=500 nm) provided in that order on the plastic substrate. The layer of xcex/2 is an equivalent film containing at least three layers and having a refractive index of from 1.80 to 2.40, and the even-numbered layer(s) of the equivalent film is a silicon dioxide layer or a layer in which silicon dioxide is the main component. For example, the even-numbered layer could be a layer of more than 80% SiO2 and the balance other metal oxides such as Al2O3. According to the invention, when the high-refraction layer of xcex/2 is a three-layered equivalent film, an optical element having good heat resistance and antireflection property is obtained. For further improving the heat resistance and the antireflection property, the high-reflection layer of xcex/2 may be made of an equivalent film of more than three layers.
To obtain good heat resistance and antireflection property, the odd-numbered layers of the equivalent film of xcex/2 contain a known high-refraction vapor-deposited substance such as titanium oxide, zirconium oxide, tantalum oxide and niobium oxide, more preferably a layer made of at least one vapor-deposited substance selected from TiO2, Ta2O5 and Nb2O5, and most preferably a layer of Nb2O5 as the vapor-deposited substance. To promote production efficiency, it is desirable that the odd-numbered layers all have the same film composition.
The resultant refractive index of the high-refraction layer of xcex/2 is in the range of from 1.80 to 2.40, the range of from 1.85 to 2.25 having better physical properties. The film constitution of the high-refraction layer of xcex/2 is so made that it satisfies the defined range of the refractive index.
According to one embodiment of the invention, the layer of xcex/4 to be formed on the high-refraction layer of xcex/2 is a silicon dioxide layer that serves as a low-refraction layer in the antireflection film. The layer of xcex/4 to be formed under the high-refraction layer of xcex/2 is usually an equivalent film of at least two layers for good antireflection property and heat resistance, but other embodiments may contain three and five layers. The film is generally constructed of a two-layered equivalent film made of a silicon dioxide layer and a layer of a high-refraction vapor-deposited substance such as titanium oxide, zirconium oxide, tantalum oxide and niobium oxide, or a two-layered equivalent film made of a silicon dioxide layer and a niobium oxide layer. To promote production efficiency, it is desirable that the raw materials for vapor deposition in forming the equivalent film of xcex/4 are the same as those for vapor deposition in forming the equivalent film of xcex/2.
Where a niobium oxide layer is used as the layer of high-refraction substance, it is preferred to use 100% niobium oxide for the vapor-deposited substance to form the layer according to an ion-assisted process.
The xe2x80x9cion-assisted processxe2x80x9d referred to herein is a well known process also called xe2x80x9cion beam assisted vapor deposition processxe2x80x9d. According to this process, a material is deposited on a substrate, such as a lens substrate, by vapor deposition using an ion plasma in a gas atmosphere, such as argon (Ar), nitrogen, oxygen, or mixtures thereof. In a common apparatus suitable to perform this process, typical vapor deposition conditions are an accelerating voltage of 100-350V, and an accelerating current of 50-150 mA. A detailed description is given in e.g., U.S. Pat. No. 5,268,781. Further details can be derived from M. Fliedner et al., Society of Vacuum Coaters, Albuquerque, N.Mex., USA.p237-241, 1995 as well as from the references cited therein.
In the ion-assisted process, argon (Ar) may be used as the ionizing gas for preventing oxidation of films being formed. Although argon is generally preferred, other ionizing gases such as oxygen and nitrogen, or mixtures of these gases could also be used. This stabilizes the quality of the films formed and enables easy control of the thickness of the films by the use of an optical film thickness meter.
A niobium oxide layer may also be formed by a method of sintering a powder containing niobium oxide, zirconium oxide and yttrium oxide and optionally containing aluminum oxide, generating a vapor of the oxide mixture from the sintered material, and depositing the vapor on a substrate. In the method of depositing the vapor on the substrate, a blend ratio of the sintered material is preferably such that niobium oxide accounts for from 60 to 90% by weight of the whole of the composition for vapor deposition, zirconium oxide for from 5 to 20% by weight, and yttrium oxide for from 5 to 35% by weight, for ensuring good physical properties of the film. In case where aluminum oxide is added, its amount is preferably from 0.3 to 7.5% by weight of the total of zirconium oxide and yttrium oxide therein.
According to another embodiment of the invention, the optical element has a basic layer provided between the plastic substrate and the antireflection film. For the basic layer, preferred is silicon dioxide or metallic niobium, and more preferred is metallic niobium. In case of the silicon dioxide layer, its film thickness is preferably from 0.1 xcex to 5 xcex to ensure appropriate film strength; and in case of metallic niobium, its film thickness is preferably from 0.005 xcex to 0.015 xcex, for ensuring the film transparency.
The advantages of the basic layer of metallic niobium are that it ensures good adhesiveness between the plastic substrate and the antireflection film, and provides an optical element that is excellent in heat resistance, impact resistance and abrasion resistance, and its absorption intrinsic to metal is low. Preferably, the metallic niobium (Nb) layer is formed in an ion-assisted process.
In the ion-assisted process, argon (Ar) is generally used as the ionizing gas for preventing oxidation of the film being formed. This stabilizes the quality of the film formed and enables easy control of the thickness of the film by the use of an optical film thickness meter.
For ensuring good adhesiveness between the plastic substrate and the basic layer and for ensuring good uniformity of the initial film morphology in vapor deposition in the ion-assisted process, the plastic substrate may be subjected to ion gun pretreatment before the basic layer is formed thereon. The ionizing gas in the ion gun pretreatment may be any of oxygen, nitrogen, Ar, or mixtures thereof. For the preferred power range, the accelerating voltage is from 50 V to 200 V, and the accelerating current is from 50 mA to 150 mA.
The antireflection film in the optical element of the invention may be formed by any suitable process. For example, it may be formed by vapor deposition, such as chemical vapor deposition (CVD) or physical vapor deposition (PVD), or by other methods such as an ion-assisted process.
The plastic substrate used for the optical substrate of the invention is not specifically defined, including, for example, methyl methacrylate homopolymers, copolymers of methyl methacrylate and one or more other monomers such as diethylene glycol bisalkyl carbonate or benzyl methacrylate, diethylene glycol bisallyl carbonate homopolymers, copolymers of diethylene glycol bisallyl carbonate and one or more other monomers such as methyl methacrylate and benzyl methacrylate, sulfur-containing copolymers, halogen-containing copolymers, polycarbonates, polystyrenes, polyvinyl chlorides, unsaturated polyesters, polyethylene terephthalates, and polyurethanes.
If desired, the optical element of the invention may have a cured film between the plastic substrate and the basic layer. The cured film may be made by curing a coating composition that comprises metal oxide colloid particles and an organosilicon compound.
The metal oxide colloid particles generally are fine metal oxide particles having a particle size of 1-500 nm. The colloid particles may be made of tungsten oxide (WO3), zinc oxide (ZnO), silicon oxide (SiO2), aluminum oxide (Al2O3), titanium oxide (TiO2), zirconium oxide (ZrO2), tin oxide (SnO2), beryllium oxide (BeO) or antimony oxide (Sb2O5). One or more of these metal oxides may be used either singly or in admixture of two or more thereof.
Several embodiments of the optical element of the invention are described by the following structures (a) to (c).